1. Field of the Invention
The present invention relates generally to inductively coupled electromagnetic energy field transmission and detection systems, such as a transponder in combination with an interrogation system, and more particularly to a method and apparatus for modulating and detecting an inductively coupled transponder subcarrier signal.
2. Brief Discussion of the Prior Art
Many objects, such as houses, pets and cars, require some means of identification. Many prior art methods of identification have required visual interrogation of the identifying medium to extract the identification data, such as reading numbers on houses, license plates on cars, and collar tags or brands on animals. Electronic identification tags have also been created, which can be associated with the object and electronically communicated with at a distance, such as the electronic sensing and actuator systems shown in U.S. Pat. Nos. 3,732,465 and 3,752,960.
The systems described in those patents are comprised of an active element, having a single transmitting and receiving coil, which operates by transmitting an electromagnetic field within the proximity of a passive electrical circuit, thereby inductively coupling the coil with the passive circuit. The passive circuit operates to create a characteristic change in the transmitted electromagnetic field that can be detected by the electronics associated with the receiving coil and used to trigger some mode of operation for the system. Although such systems remove some of the restrictions associated with the previously described visual identification systems, such systems are really nothing more than electronic keys, and actually convey less information to the active element than does a visually inspectable tag.
More sophisticated electronic systems use an exciter coil to transmit a high intensity electromagnetic energy field in the proximity of an electronic tag or transponder. The transponder is energized by the electrical energy inductively transferred by the transmitted magnetic field and is made operative to output a modulated identification signal which can be detected by an inductively coupled receiving coil proximately located at the exciter coil. The passive tag or transponder element of many of these devices, such as that described in U.S. Pat. No. 4,262,632, includes a coil which receives electromagnetic energy from a transmitted interrogation signal and retransmits an identification signal developed by the identification circuitry associated with the transponder. Electrical control circuitry within the transponder converts and rectifies the energy received from the transponder coil and develops a dc power source for use in operating the transponder's identification circuitry.
Transponders which utilize a sufficiently large capacitor or resident power source, such as a battery, are able to transmit identification signals over fairly large distances, from a few yards to thousands of feet. However, in certain applications, such as identifying small animals, the size of the sealed packaging required to house the battery or capacitor may be too large for the intended use.
A great amount of effort has been expended to develop a truly passive transponder, which will do more than simply operate as an electronic key. A transponder which can be safely implanted within livestock and interrogated from a practical distance would help to reduce problems associated with certain regulations being implemented by the European Economic Community that will require all livestock to be separately identified.
One effort to create such a transponder resulted in the syringe-implantable transponder of European Patent No. 258,415. This patent and other related patented systems, such as U.S. Pat. Nos. 3,859,624, 3,689,885, 4,532,932 and 4,361,153, disclose passive elements which operate in real time and therefore do not require any type of significant energy storage means. The transmitting and receiving units of these systems, which are often referred to as interrogators or readers, typically include either a single, dual or triple coil arrangement, which is used to both transmit a high intensity electromagnetic field in proximity of the identification unit and receive an identification signal retransmitted by the transponder in proximity of the reader coils of the interrogator.
The detection range of such systems is typically very restricted since the strength of the electromagnetic field produced by the transponder drops by 1/d.sup.5 at the receiver, where d is the distance between the receiving coil and the transponder coil, as the interrogator is moved away from the transponder. In fact, it has been calculated that at distances of 6 to 8 inches the strength of the magnetic field carrying the identification signal from the transponder, in devices similar to that described in the European Patent No. 258,415, is only an estimated one billionth the strength of the magnetic field carrying the interrogation signal to the transponder. Hence, the interrogator must normally be placed in very close proximity to the transponder in order for the identification signal to be detected. This limitation, of course, greatly restricts the utility of such devices, since not all objects may be so closely approached in order to be read.
Although the specification of European Patent No. 258,415 states that the system disclosed therein is operative to detect the retransmitted signal at distances on the order of inches, it has been found that devices constructed in accordance with that specification are actually incapable of obtaining such a range and are generally only effective when positioned within about an inch of the transponder. An identification device which is limited to operation within such a small range is of limited usefulness, especially when it is to be used to identify large or wild animals or other objects which cannot be readily approached.
If a transponder system is to be expected to maintain a greater reading distance, a number of different characteristics of the system must be considered. To increase or maintain a certain reading distance, the interrogator's ability to detect the identification signal transmitted by the transponder must be enhanced. A limitation restricting the reading distance of the transponder system is associated with the strength of the signal transmitted from the transponder to the reader. Hence, it is not only important to assure that an efficient inductive couple exists between the interrogator and the transponder, so that the transponder receives as much energy for operation as is possible, but it is also important to maximize the strength of the signal transmitted from the transponder to the interrogator.
Prior art transponder systems not only generally fail to provide for an efficient coupling between the interrogator and the transponder, but also fail to maximize the strength of the transponder signal. One reason prior art systems do not maximize the transponder signal is because the physical and electrical configurations of the interrogator's transmitting and receiving coils require that the frequency of the transponder signal be significantly different from that of the interrogator signal, thereby limiting the frequency and amplitude of the subcarrier signal. For example, U.S. Pat. No. 3,689,885 states that "[i]t has been found that by having the carrier time-base signal at a comparatively high frequency such as, for example f.sub.z =450 KHz, and the electromagnetic power field at a lower frequency, for example, f.sub.1 =50 KHz, interference between the electromagnetic coded information field and the electromagnetic power field is minimized." Col. 7, lines 11-17.
The efficiency and speed of many of the prior art transponder systems is also restricted by the type of modulation technique used to modulate the subcarrier. Many prior art systems use frequency-shift-keyed modulation for modulating the transponder's subcarrier, which generally requires fairly complex circuitry and which severely limits the data transfer rate of the subcarrier. U.S. Pat. No. 3,964,024 discloses a transponder system which utilizes phase-shift modulation for modulating the subcarrier, a technique which reduces the complexity of the circuitry required in the transponder, generally enhances the efficiency of the band-width, and increases the data rate of the subcarrier. However, this modulation technique requires that the transponder be equipped with two separately located coils, one for receiving power from the interrogator and one for transmitting the identification signal. In addition, since the data bit shifts high or low each time a phase change occurs, this technique does not anticipate a change in the orientation of the transponder resulting in a phase shift of the incoming signal or the transmitted signals.
Hence, a need has arisen for a transmission and detection system which can simultaneously transmit a high energy magnetic field, sufficient to power the transponder unit, and detect a localized retransmitted magnetic field at greater distances and with greater reliability.